Methylvinyl ketone (MVK), ethylvinyl ketone (EVK), and 2-cyclohexene-1-one (CHX) were nominated and selected for study by the NTP as part of a class study on alpha,beta-unsaturated ketones. MVK is a representative noncyclic member of the major class of straight-chain aliphatic alpha,beta-unsaturated ketones. EVK is a secondary conjugated carbonyl compound from the subclass of aliphatic alpha,beta-unsaturated ketones. CHX is a prototype of the cyclic alpha,beta-unsaturated ketones. Prechronic Inhalation studies of these ketones are being conducted to characterize and compare the relative toxicities and also to facilitate the design of chronic inhalation studies of these chemicals. Following completion of prechronic studies, mechanistic studies will be considered, and if needed, two-year bioassays will be performed through NTP contracts. This chemical class study will allow more accurate prediction of the potential toxicity and carcinogenicity of other untested alpha,beta-unsaturated ketones. Prechronic (14- and 90-day) studies of CHX and MVK have been completed, and data are being evaluated. Methods are being developed for generation and monitoring of EVK. EVK will be studied using the same designs used to study CHX and MVK.